Semiconductor optical devices such as modulators, lasers, and photodetectors often incorporate a p-i-n structure. As is well known, such structures consist of p and n regions separated by a very lightly doped intrinsic region of one or more semiconductor materials. The intrinsic region may, for example, comprise a single layer of silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or aluminum gallium arsenide (AlGaAs). The intrinsic region of an optical modulator, for example, typically comprises a multiple-quantum-well (MQW) region that includes multiple well and barrier layers.
In devices of the aforementioned type, the lower or buried layer of the p-i-n structure is often epitaxially grown on the uppermost surface of a reflective mirror structure. When a bias voltage is applied to the p-i-n structure, light reflected by the mirror structure is absorbed by the quantum wells in the intrinsic region. As will be readily appreciated by those skilled in the art, the capacity of an optoelectronic device to imprint information optically is a function of this ability to alternately absorb and admit light incident thereon with changes in bias voltage.
The focus of conventional MQW device fabrication techniques has, heretofore, been limited to minimizing defects such as deep traps, recombination centers, interstitials and vacancies, and chemical impurities. While these considerations are important, there are other factors which arise during the fabrication process which may affect the optical properties of such devices. The intrinsic regions of MQW devices are engineered with atomic scale layering to optimally exploit their optical properties. It is known that surface roughness of a single monolayer height can substantially degrade the optical properties of structures utilizing atomic layering. In fact, where surface roughness exceeds several atomic layers, an MQW device may be rendered totally inoperative. It is therefore highly desirable to minimize defects and, in particular, roughness which occurs on an atomic scale.
The atomic smoothness/roughness of the buried p-i-n conductive layer will necessarily affect the surface characteristics and atomic ordering of the subsequently grown intrinsic region. Previous investigations, however, have suggested only that the intrinsic layer of the p-i-n structure should be grown on an n-type layer ("n-down") rather than on a p-type layer ("p-down"). In fact, a review of the technical literature in this field reveals few examples of a p-down p-i-n structure and, even where such structures are reported, the p-down region is employed for purposes other than atomic scale roughness (e.g., controlling the band structure of the device). Thus, heretofore, no one has sought to promote smooth atomic ordering of the MQW intrinsic region of a p-i-n structure by controlling the surface characteristics of the buried conductive layer thereof.